The present invention relates generally to a finder optical system comprising an image inversion optical subsystem, and more particularly to a finder optical system for viewing an image of an object inverted by an objective in the form of an erected image using an image inversion optical subsystem.
Typical finder optical systems are known from U.S. Pat. Nos. 3,810,221 and 3,836,931, JP-A's 59-084201, 62-144127, 62-205547, 1-257834 (corresponding to U.S. Pat. No. 5,274,406), 8-201912, 8-234137, 8-248481, 8-292368, 8-292371 and 8-292372, and EP 0722106A2.
These prior art systems include a single image formation type real image finder optical system used on cameras or video cameras, which makes use of an image inversion optical subsystem comprising an elaborately combined mirror and prism arrangement for the observation of an erected image. This optical system is not only designed to erect an inverted image of an object by use of a combined mirror and prism, but is also designed to achieve size reductions by using a turn-back optical path. Since the reflecting surfaces of the mirror and prism in the image inversion optical subsystem are decentered with respect to the optical axis, however, rotationally asymmetric decentration aberrations are produced when the reflecting surfaces have powers. The resulting optical performance loss can never be prevented only by use of a rotationally symmetric lens. For this reason, optical elements such as mirrors and prisms in image inversion optical subsystems are generally in plano surface forms.
To meet recently increasing demands for size reductions of cameras and video cameras, it is now required to achieve further size reductions of finder optical systems used on them. To this end, various investigations are made of the direction and angle of the reflecting surfaces of mirror and prism elements in image inversion optical subsystems or combinations of such optical elements. However, any drastic solution to this problem cannot be obtained because such reflecting surfaces are still used in plano surface forms.